also edit /etc/inittab
and # out
T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
so it looks like
#T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
this is probably the last line in the file

you need an IP that is not used by your network or anywhere else for that matter
e.g.
192.168.36.1 on the server and 192.168.36.2 on the client

on the server machine
create a file called
/etc/ppp/options.ttyAMA0
and fill it with
noauth
nocrtscts
xonxoff
passive
local
maxfail 0
192.168.36.1:192.168.36.2 #note this is opposite to below

and the client
create a file called
/etc/ppp/options.ttyAMA0
and fill it with
noauth
nocrtscts
xonxoff
passive
local
maxfail 0
defaultroute
persist
192.168.36.2:192.168.36.1 #note this is opposite to above

having done this again over the weekend at a mates he's a couple of other things that you really need to do to get it working properly
...

on the server in the
options file add
proxyarp [this means that other hosts can find the computer on the end of the ppp line]
and edit /etc/sysctrl.conf
and enable
net.ipv4.ip_forward=1
this means packets can get back OK
apt-get update && apt-get upgrade takes a while over a serial link but works

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You might want to try using some better cable rather than just unshielded straight wires - you might get better results with UTP/STP network cable, or possibly using some ribbon cable with a conductor between the rx/tx conductors that's grounded at one end (similar idea to 80 conductor PATA cables). I guess line noise will be the biggest enemy at higher baud rates, but I'm no expert on the matter - most of the time I use serial cables I'm either using 9600 or 115200. (fine for terminal connections)

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You will get significantly higher speeds if you series-terminate the driver (i.e. TXD) on each Pi with a 33 ohm resistor.

Insert a 33 ohm resistor between the TXD pin and the interconnecting line. To prevent crosstalk you can either use a twisted pair, with the A in each line tied to ground and the B line for signal - or coax cables.

It'd be interesting to see how fast you can get inter-Pi communication over the UART.

I can set the UART block input clock (config.txt init_uart_clock) up to 250MHz before everything breaks: however this high clock rate is a bit pointless due to other limitations. It appears you can't set the UART baud rate in linux above 4,000,000bps - setting it to 4,000,001 gives "invalid argument".

Even so, I can't test yet at 4Mbaud - FT232R devices only go up to 3Mbaud. I'd have to connect two Pis back to back and set both up.

Gert van Loo wrote:If you really want distance go for current drivers. (I think it is RS424) instead of RS323.
You can then do a mile or more.
Always when using long wires beware if lightning spikes pickup!

RS-422 is the go-to for duplex transmission over long distances (>3 metres or so) - differential transmit/receive also lend themselves nicely to using standard CAT5/5e cables. Converter ICs are also readily available and cheap, and need little interfacing.

With the PPP approach described above, you could get a modest 4Mbps "network" connection to a Model A via the UART. I shall poke around and see why the 4Mbaud limit exists.

updated for sanity
though not tried with 33ohm resistors yet will see if I can get some

Model B is server and connected to the local network
Model A is client and connected to the model B via the GPIO serial Lines
see http://elinux.org/images/2/2a/GPIOs.png for GPIO
connect pin 6 on both machines together [GND]
connect pin 8 [GPIO 14 TXD] to Pin 10 [GPIO 15 RXD] on the other machine - and visa vesa

on both machines edit
/boot/cmdline.txt
and remove the ttyAMA0 bits
so

I've been using a USB serial converter to connect my RPI2 B+ to my RPI A+ , both are runing Rapicams out in the field via Power over Internet adapters to the B+. This worked OK but I wanted more speed so I could transfer image data from the A+ back to my PC.

I've tried Model A to Model B via serial lines as stated here (indoors not in the field) and followed the instructions as from "updated for sanity Wed Jun 25, 2014 1:29 pm" above but could not get it to work. The only changes I made were:

a) My router will only allow an ip range 192.168.0.2 to 192.168.0.254 so in the /etc/ppp/options.ttyAMA0 file I used 192.168.0.40:192.168.0.41 for A+ and 192.168.0.41:192.168.0.40 for B+.

b) The Pi's did not have a PPP directory in /etc/ so I created one and gave me Group and Pi access.

I tried a sh log in to both 192.168.0.40 and 192.168.0.41 also tried to ping them both but no joy.

I did a "sudo apt-get update && sudo apt-get upgrade -y" and "sudo apt-get dist-upgrade", then double checked all the file changes and the wiring as in the instructions. I then did this on an RPI B+ instead of the RPI2. Still no joy.

mshannon wrote:I'v...
b) The Pi's did not have a PPP directory in /etc/ so I created one and gave me Group and Pi access.
...?

it probably means something is not installed
I'm not in a position to try ATM
but
1) get this working in a lab first
2) try installing pppd umm
remove the fiels you have created in ppp
sudo apt-get install ipppd #? possibly
then see if they exist
...

I'll see if I can re- do these instructions with later version of raspbian [but it will not be this week]

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